KEYS: linux/key-type.h needs linux/errno.h

KEYS: linux/key-type.h needs linux/errno.h

linux/key-type.h needs to #include linux/errno.h as it refers to ENOKEY.
Without this, with sparc's allmodconfig in one of my test trees, the following
error occurs:

include/linux/key-type.h: In function 'key_negate_and_link':
include/linux/key-type.h:122:43: error: 'ENOKEY' undeclared (first use in this function)
include/linux/key-type.h:122:43: note: each undeclared identifier is reported only once for each fun

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>